The invention relates to an injection means for an injection moulding machine with a screw for metering plastics which is mounted in a rotatable manner in a clamping plate, which is longitudinally displaceable by means of at least two electrically driven spindles, wherein the pressure exerted upon the screw by the plastics is determined by at least one sensor which measures the deformation of a connecting ring arranged between a stationary sleeve surrounding the pivot bearing of the screw and the clamping plate.
With hydraulically driven injection moulding machines, one can assume that the hydraulic pressure translates into the pressure prevailing in the plastics. The pressure in the plastics can consequently be worked out from the hydraulic pressure, which renders the use of expensive and vulnerable sensors in the moulding space superfluous. With electrically driven injection moulding machines, on the other hand, it was considered for a time that the torque supplied by the drive, which determines the pressure in the plastics would be satisfactorily determined by the instantaneous current intensity. Actually, however, energy is periodically taken up and given out by the motor itself, so although by monitoring the current intensity an average torque and thereby an average pressure in the material can be determined, monitoring the injection procedure simply by monitoring the current taken up by the motor is problematic.
For the reasons described, it was realised relatively early on that in order to monitor the injection and retention pressure, the pressure occurring in the screw or the pressure exerted by the screw had to be measured (see JP 60-174625 Nissei, EP 0 230 488 A1). With an apparatus of the type defined in the introduction, as is shown, for example, in EP 0 260 328 A1, there is a problem in the configuration and arrangement of the sensors in that the injection and retention pressure results from the application of a force which, although less than the mould closing pressure, can easily be greater than 100 kN, even with average-sized machines. As this pressure must be taken up by the connecting ring between the stationary part of the pivot bearing of the screw and the clamping plate, without the precise orientation of the screw suffering, it has been assumed that this connecting ring is constructed in a solid manner and has to be arranged in the axial direction between the bearing sleeve and clamping plate. In particular, this connecting ring was constructed integrally with the bearing sleeve of the pivot bearing of the screw (see JP-7-67722 Niigata).
The known arrangement of pressure sensors has the disadvantage that the back pressure that occurs during plasticizing and metering of the plastics is an order of magnitude less than the injection and retention pressure, and the deformation of the connecting ring, configured in a known manner, between the bearing sleeve and clamping plate does not deliver a satisfactorily clear signal. The invention avoids this disadvantage in that the connecting ring is configured as an annular disc, the inner edge of which is connected to the sleeve and the outer edge of which is connected to the clamping plate.
The disc-shaped construction of the connecting ring deformed by the pressure of the plastics firstly results in a considerable relative displacement of the screw and clamping plate, when the screw is acted upon by means of the clamping plate. The displacement represents the measurement which can be easily registered by means of expansion measuring strips fitted in the annular disc itself, or by known apparatuses for monitoring the relative position of two machine parts. This is particularly the case when the cross-section of the annular disc is reduced in the central area. In the peripheral and radial direction, the annular disc according to the invention has great stability, so despite its considerable deformation in the longitudinal direction, the precision of the shaft mounting is not affected.